Shrink fitting of an annular sleeve in a corresponding outer housing opening is a very old and well-known art. It has been used in making a valve cartridge type housing having an annular sleeve provided with a precisely honed bore to slideably receive a valve spool, for example.
Instead of the very old practice of casting recesses in the annular bore opening in the housing. It was viewed easier to machine annular recesses in the outer surface of a sleeve which is then shrunk fit into an outer housing. The recesses accommodate fluid communication between the aligned radial ports in the sleeve and housing. This latter practice is considered reliable when the housing and the sleeve are made of the same metal or metals having nearly an identical co-efficient of expansion, such as cast iron and steel. It is considered unsatisfactory however, if dissimilar metals are used, such as aluminum and steel for example.
Even though aluminum is a preferred housing material because of economics and its resistance to corrosion, temperature changes encountered in many applications can result in deflection of the sleeve bore in magnitudes greater than the close fit tolerance between the sliding valve spool and the bore to cause the spool to become locked or frozen in the bore.
Therefore, in most applications, it has become a general practice to use the popular aluminum housing with a steel sleeve wherein the sleeve is concentrically stepped along its axial length and O-rings are used between valve passages and the annular recesses formed between the concentric steps in the sleeve and the annular opening in the housing. The sleeve is then held in its axial position by a mechanical shoulder and end plug arrangement.
It would be particularly advantageous to use a shrunk fit sleeve and housing of dissimilar metals to take advantage of the characteristics of aluminum as a housing material and a hard metal such as steel for the valve spool sleeve. However, the unreliability related to substantial deflections of the sleeve bore when subjected to significant temperature changes represents a problem which has not been satisfactorily solved by those skilled in the art and has discouraged use of such combinations in a shrunk fit configuration.